1. Field of the Invention
The present invention relates to a wet electrophotographic printer using a high density liquid developer, and in particular, to an apparatus for preventing wraparound in a wet electrophotographic printer which can prevent developer wraparound in a developing roller and a photosensitive body during development.
2. Description of the Related Art
In general, an electrophotographic printer obtains a desired image by forming an electrostatic latent image on a photosensitive medium or body such as a photosensitive belt or drum, developing the electrostatic latent image by using developers having predetermined colors, and transferring the developed image to paper. The electrophotographic printer is divided into a wet type and a dry type according to a kind of developer. The wet electrophotographic printer employs a liquid developer obtained by mixing a volatile liquid carrier with a powder type toner.
Since the wet electrophotographic printer using the liquid developer utilizes a toner having a grain size below about 0.5 to 5 μm, the wet electrophotographic printer obtains higher quality image than an image formed by the dry electrophotographic printer using the powder type toner in development of the electrostatic latent image, and prevents damage due to harmful toner dusts. Therefore, the wet electrophotographic printer has gradually become popular.
However, the wet electrophotographic printer obtains an appropriate image density by using a low density liquid developer, the density generally being below 3% solid. Accordingly, a complicated developer delivery system to supply sufficient liquid developer to a developing region of a developing apparatus and collecting the developer is required to obtain the appropriate image density, thus increasing a bulk of the developing apparatus and complicating the system.
In addition, an apparatus to control a density of the liquid developer is necessary to obtain the appropriate image density when replenishing the developer due to variations of toner grains after the development.
Therefore, the developer delivery system needs to be removed or simplified to prevent the wet electrophotographic printer from being oversized or excessively complicated. Accordingly, the wet electrophotographic printer that uses a high density liquid developer over 3% solid, rather than a low density liquid developer below 3% solid, has been more polular.
FIG. 1 is a schematic view illustrating a general electrophotographic printer using a high density liquid developer.
The wet electrophotographic printer includes a photosensitive body 9 such as an organic photoconductive (OPC) drum, a laser scanning unit 11, a charged roller 12, a developing apparatus 13, a transfer belt 10 moving on a caterpillar-like photosensitive belt, primary and secondary rollers 21 and 22 for rotating the transfer belt 10 on the photosensitive belt, a first transfer roller 8 for transferring an image to the transfer belt 10, a second transfer roller 23 for transferring the image to paper P, a fixing roller 25 for fixing the image, and a cleaning blade 51 for removing a residual image from the transfer belt 10. The constitutional elements of the printer sequentially perform an image formation process including charging, exposure, development, transfer and fixation by mutual operations, to form a wanted image on the paper P.
In general, a color printer includes four laser scanning units 11 for color printing, and four developing apparatuses 13 for individually containing high density liquid developers 48 of 3 to 40% solid, which have four colors, namely black, yellow, cyan and magenta.
As illustrated in FIG. 2, each of the developing apparatuses 13 includes a storing unit 6 for storing the high density liquid developer 48, a developing roller 7 positioned below the photosensitive body 9, a deposit roller 14 installed below the developing roller 7 for transmitting an electric force to the liquid developer 48 to form a charged developer layer on the developing roller 7, a metering roller 15 for transmitting a predetermined voltage to the charged developer layer formed on the developing roller 7 by the deposit roller 14 to adhere a large volume of toner to the developing roller 7, and restricting the charged developer layer to a predetermined toner amount or density (% solid) to be supplied into a nip between the developing roller 7 and the photosensitive body 9, and a cleaning roller 16 for cleaning the developing roller 7.
The deposit roller 14 and the metering roller 15 serve to supply a predetermined toner amount or density of developer layer into the nip between the developing roller 7 and the photosensitive body 9, regardless of a density of the high density liquid developer 48 having 3 to 40% solid, or the liquid developer 48 having a density that varies during use.
The operation of the wet electrophotographic printer is explained below.
According to a print command, an electric charge layer, namely an electrostatic latent image corresponding to an image to be printed is formed on the photosensitive body 9 by the charged roller 12 and the laser scanning unit 11. A predetermined amount of toner of the developer layer formed on the developer roller 7 is adhered to the electrostatic latent image region from the liquid developer 48 of the storing unit 6 by the deposit roller 14 and the metering roller 15, thus forming a toner image.
Here, the liquid developer 48 is formed as a charged developer layer on the developing roller 7 due to an electric force from the deposit roller 14, and formed as a predetermined toner amount of a developer layer on the developing roller 7 due to a predetermined voltage from the metering roller 15.
The image formed on the photosensitive body 9 by the developing apparatus 13 is first transferred from the photosensitive body 9 to the transfer belt 10 due to a voltage and pressure of the first transfer roller 8 positioned inside the transfer belt 10. Since the transfer belt 10 is rotated by the primary and secondary rollers 21 and 22, the image transferred to the transfer belt 10 is moved to the second transfer roller 23 and then is transferred to the paper P due to a voltage and pressure of the second transfer roller 23.
The image transferred to the paper P is fixed to the paper P by the fixing roller 25 and a fixing backup roller 26, thus forming a desired image.
After the image transferred to the transfer belt 10 is transferred to the paper P, the transfer belt 10 is continuously rotated by the primary and secondary rollers 21 and 22 and moved to the cleaning blade 51 installed to contact the image formation side of the transfer belt 10 at the side of the primary roller 21. Here, residual developer sediments (generally 90 to 98% of developer is transferred to the paper) are removed from the surface of the transfer belt 10 by the cleaning blade 51 for succeeding image printing, and collected in a used developer storing unit 52.
The transfer belt 10 from which the residual developer sediments are removed repeats the aforementioned procedure to perform the succeeding electrostatic latent image formation and development through the photosensitive body 9, the laser scanning unit 11 and the developing apparatus 13.
However, as shown in FIGS. 3A to 3C, the conventional printer has a structure wherein the outer surfaces of the metering rollers 15, 15′ and 15″ for varying the charged developer layers formed on the developing rollers 7, 7′ and 7″ by the deposit roller 14 into the predetermined toner amount or density of developer layer and the outer surfaces of the developing rollers 7, 7′ and 7″ touch each other.
Accordingly, as shown in FIG. 4, in a state wherein the charged developer layer is formed on the developing roller 7 by the deposit roller 14, when the metering roller 15 forms the nip with the developing roller 7 and restricts the charged developer layer to have a predetermined toner density by rotation, the liquid developer 48 pushed toward both side surfaces of the metering roller 15 is collected in the space between the metering roller 15 and the developing roller 7 where the developer layer 17 restricted to have a predetermined density by the nip between the developing roller 7 and the metering roller 15 is positioned. At the worst, the liquid developer 48 is transferred to the photosensitive body 9 through the developing roller 7 to form wraparound 18, thus partially or wholly supplying the toner over a predetermined amount or density into the nip between the developing roller 7 and the photosensitive body 9.
As described above, the toner over a predetermined amount or density supplied into the nip between the developing roller 7 and the photosensitive body 9 reduces a quality of the image. In addition, after the image is transferred to the paper P, the toner is left on the surface of the photosensitive body 9, increasing the developer segments collected in the used developer storing unit 52 by the cleaning blade 51. Therefore, consumption of the developer is unnecessarily increased.
Referring to FIG. 5A through FIG. 5C, a developing apparatus of another conventional wet electrophotographic printer is illustrated using a squeezing unit squeezing a developer transferred on a latent image region of a photosensitive body to change a toner contained in the developer into an image film and to remove a carrier except for the toner contained in the developer when excess developer is supplied on the photosensitive body. The developing apparatus is disclosed at Korean patent laid-open No. 2000-56601 of Lee youn Keun et al., filed on Feb. 24, 1999 and laid open on Sep. 15, 2000 in the title of “A Squeezing Unit of Wet Electrophotographic Printer”.
In the developing apparatus, the squeezing unit 27 comprises a squeeze roller 28 disposed to come in contact with a photosensitive body 9′ such as a photosensitive belt and away from a developing roller 7′″ to be driven by the photosensitive body 9′ during development, and a blade 30 disposed to be separable from squeeze roller 28 to remove a drip line 33 (FIG. 5C) forming on the photosensitive body 9′ when the squeeze roller 28 is rotated in a direction opposite to a rotation direction of the photosensitive body 9′ to remove the developer remaining on the photosensitive body 9′ after development.
The squeeze roller 28 has taper portions 29 formed at both ends thereof to prevent a wraparound generating between the blade 30 and the squeeze roller 28 when the drip line 33 is removed by the blade 30, from being transferred to the photosensitive body 9′.
However, in the developing apparatus of the conventional printer, the squeezing unit 27 needs a separate blade 30 and is disposed away from the developing roller 7′″. Also, the photosensitive body 9′ is formed of a photosensitive belt. Accordingly, the printer is oversized or excessively complicated.